The present invention relates to a NAS system equipped with a snapshot function, and a difference snapshot remote copy method.
In the related art, one function of a NAS (Network Attached Storage) system is a so-called “snapshot” function of logically holding a data image for a primary volume (a logical volume for storing user data) at a point in time when a snapshot generation instruction is received. A snapshot function is utilized when data of a primary volume is erased due to human error or when it is wished to restore a data image for a primary volume at a desired point in time, etc.
A difference snapshot is disclosed in Japanese Patent Laid-open Publication No. 2004-342050 (US Unexamined Patent Publication 2004/0186900). According to a difference snapshot, a data image for a primary volume at a certain point in time in the past is logically restored using information indicating updated data of the primary volume for after the point in time the snapshot generation instruction is provided and the update position of this data. A difference snapshot therefore has the advantage that it is possible to maintain a data image for a primary volume at the point in time designated for snapshot generation using a lower storage capacity compared with the case where a data image for an entire primary volume is stored as is.
However, it is not possible to restore a data image for a primary volume by utilizing a snapshot when there is damage to a storage system equipped with a snapshot function. A remote copy system taking this situation into consideration where difference data for a snapshot acquired by a first NAS system arranged at a primary site is remote copied to a second NAS system installed at a secondary site and a snapshot is also maintained at the second NAS system so that when a fault occurs at the first NAS system, a data image for the first NAS system is restored using a snapshot of the second NAS system is also well-known.
In the remote copy system, the first NAS system acquires a snapshot based on a time preset by the user, and difference data for this snapshot is remote copied to the second NAS system. A plurality of file systems are installed at the first and second NAS systems. Difference data for snapshots for the plurality of file systems installed on the first NAS system is respectively remote copied to the respective plurality of files systems installed on the second NAS system. The file system for the remote copy source and the file system for the remote copy destination correlate, with the relationship between the two being referred to as a “pair”.